A cellular radiotelephone operating in a cellular communications system must identify itself to the system servicing the radiotelephone, allowing the system to interface the radiotelephone to the landline telephone system. The identification typically informs the system that the radiotelephone is active in that particular system in addition to telling the system the telephone number assigned to the radiotelephone. This identification is accomplished when the radiotelephone is powered up and receiving but not transmitting, in other words, in an idle state.
Autonomous registration is such an identification method (U.S. Pat. No. 4,775,999 to Williams, assigned to Motorola, describes such a registration method). This registration is performed at various time intervals to indicate that the radiotelephone is active in the system. The time interval is determined by the system's master clock and the next registration count maintained by the radiotelephone.
The master clock value is transmitted by the cellular system, at regular intervals, to all the radiotelephones being operated within the system via the registration identification message. This clock, counting at a set rate, is typically the same throughout the entire cellular system within a metropolitan area.
The radiotelephone's next registration count is a threshold count used to determine when the next autonomous registration will occur. When the radiotelephone receives the master clock count from the system, it compares the clock count to the next registration count. If the clock count is equal to or past the next registration count, a register condition is detected and the radiotelephone will attempt to access the system to accomplish the registration process. A number of steps, as illustrated in FIG. 1, must be performed for a successful registration.
First, the radiotelephone receives the master clock data from a channel and compares it to the next registration threshold value (A) to determine if registration should be performed. This process is continued until the master clock value equals or exceeds the threshold.
Second, the radiotelephone rescans (F) the channels to determine if it still has the best channel. Due to the mobility of the radiotelephone, it may have moved out of one cell's coverage area into another since the last scan. The quality of the channel's signal is determined by signal strength.
Third, the radiotelephone attempts (B), for a predefined number of times until successful, to seize the best channel. Every time the radiotelephone attempts seizure it turns on its carrier and typically transmits its telephone number to the system. The system may also require the radiotelephone to transmit its serial number at this time. This step is a handshaking process between the system and the radiotelephone.
Fourth, the radiotelephone waits (C) a predetermined length of time for the system to acknowledge that it correctly received the registration information from the radiotelephone. A typical length of time the radiotelephone waits is five seconds. If the system answers back, within this time, that it correctly received the information from the radiotelephone, a successful registration has occurred.
Upon a successful registration (D), the next registration threshold in the radiotelephone is replaced by the master clock count that triggered the registration process added to an offset. This count, that varies between different systems, is typically an hour. Once the threshold is updated, the radiotelephone returns to the idle state until the next master clock count that passes the next registration threshold is received, causing the entire registration process to occur again.
If the registration is not confirmed, the next registration threshold is replaced by the master clock count that triggered the registration process typically added to a random offset (G). Once this is done, the radiotelephone returns to the idle state, waiting for the next registration.
The problem with the present method of registration occurs when the radiotelephone is unable to successfully seize a channel (B). Unsuccessful seizure can occur in poor cellular coverage areas. If this situation arises, the radiotelephone will repetitively attempt to seize a channel upon receipt of the next master clock value, typically within eight seconds of the last failed attempt. This occurs because the next registration threshold is not updated with a new threshold. In this case, the old threshold is being compared to the new master clock value received and the new clock value will always exceed the next registration threshold, therefore requiring another registration attempt immediately.
During each attempt to seize the channel the radiotelephone is turning on its transmitter to transmit its telephone number to the system. Each time the transmitter is turned on, extra power is required to drive it. This is typically not a problem with mobile radiotelephones where the vehicle electrical system provides virtually limitless power. In battery powered portable and transportable radiotelephones, however, these repetitive seizure attempts will increase battery drain, thereby shortening the operation time of the radiotelephone's battery.
There is therefore a need for a method of autonomous registration that increases the length of time between a radiotelephone's failed registration attempts, thereby reducing the power drain on battery dependent radiotelephones.